The present invention relates generally to the control of flexible, undamped or lightly damped structures, and more particularly, the present invention relates to the use of electrorheological fluids as a structural component to obtain controllable structural behavior in extended mechanical systems such as plates, panels, beams and bars or structures including such elements.
Generally, the stiffness and damping characteristics of a mechanical structure are fixed parameters which cannot be easily changes or controlled once fabrication is complete. This is particularly the case with extended mechanical structures in which the damping and stiffness parameters are distributed throughout the structural materials in contrast to lumped parameter systems in which the damping and stiffness are concentrated in a limited number of discrete elements. Distributed control of stiffness and damping are desirable and often required in dynamic mechanical systems in which system motion may result in bending or flexing of the structural elements. Controllable structural behavior of the present type is particularly desirable in numerous engineering applications, which include but are not limited to the control of vibrations in aerospace and automotive applications, sound propagation through panels and walls, flexible fixturing in advanced manufacturing systems, and improvement of robot manipulator response time.
The interaction of dynamic forces with mechanical structures generally results in the generation and propagation of bending (flexure) waves in the structure. Bending waves travel easily along a structure and around corners. If the induced bending waves are sufficiently intense, the material of the structure may fail, or electronic or mechanical equipment attached to it may malfunction. Bending waves of less intensity may be strong enough to radiate disturbing sound or to cause unacceptable vibration. Bending waves are easily excited in plates, panels, beams and bars by air-borne or water-borne sound waves. In turn, bending waves readily radiate sound energy into fluid media.
In aerospace applications, for example, a fuselage or other structure can be set into vibration (1) by sound waves produced by the engines or propellers, (2) by direct excitation from the vibration of the engines, or (3) by traveling turbulence vortices moving over the exterior surfaces due to motion of the craft through the air. Such structure-borne bending waves in the fuselage radiate air-borne sound into the cabin and cause vibrations of equipment.
To alleviate these and other undesired phenomena in aircraft and other structures, methods for controlling or reducing the amplitude of the bending waves must be found. The variable control of structural behavior which is desired is to be distinguished from traditional damping means, which typically operate as discrete couplings between the elements to be isolated and the source of mechanical disturbance. While a large variety of existing damper elements can successfully control the motion of coupled members in many cases, discrete dampers are inherently inadequate to control the overall constitutive characteristics of structures in the manner contemplated herein. Spatially discrete damping treatments are only capable of controlling a limited number of vibrational modes. It would therefore be desirable to achieve a system that allows for control of spatially distributed parameters which can in theory control an infinite number of vibrational modes.
In certain types of fluid mount and damper applications, electrorheological fluids have provided remarkable results in system control. Electrorheological (ER) fluids are materials which change their mechanical properties in the presence of an electric field. For example, see U.S. Pat. Nos. 3,047,507; 4,129,513, and 4,772,407. In general, ER fluids consist of a suspension of very fine particles in a dielectric liquid media. Such fluids were first referred to as "electroviscous" because of their apparent viscosity changes in the presence of an electric field. A better understanding of these types of compositions has revealed that the phenomenon being observed is a change in the minimum stress required to induce flow in the fluid, while the actual viscosity remains generally constant. Accordingly, these effects are better described in terms of the total rheology of the composition, and as such are now more commonly referred to as "electrorheological" fluids. In the absence of an electric field, ER fluids exhibit Newtonian flow characteristics; their strain rate is directly proportional to applied stress. However, when a sufficient electric field is applied, a yield stress phenomenon occurs such that no flow takes place until the stress exceeds a yield value which rises with increasing electric field strength. Because electrorheological fluids change their characteristics very rapidly when electric fields are applied or removed, they possess great potential for providing rapid response interface in controlled mechanical devices. Typically, ER fluids have been utilized in mechanical systems such as electromechanical clutches, fluid filled engine mounts, high speed valves and active dampers.
British Pat. No. 1,259,802, for example, teaches the use of an electroviscous or magnetoviscous fluid within a damper or mount. The fluid is provided between opposing walls of a cavity in the mount member. The mount member is coupled between load elements to control the motion condition therebetween. Other representative damper members or mounts include U.S. Pat. Nos. 3,207,269; 4,720,087 and 4,733,758.
Control of the overall dynamic properties of structures is not easily or efficiently accomplished by localized damping, and in many cases cannot be accomplished to the extent desired by localized damping. Even for a simple plate-like structure of finite size there are an infinite number of frequencies at which resonance can occur. For each resonance there is a different arrangement of nodal lines and points of maximum vibration over the surface of the plate. Attempts to control structure-borne vibration in applications such as aircraft include that disclosed in U.S. Pat. No. 2,361,071. Electronic actuator devices are placed at locations within the structure which produce amplified, tuned vibrations which responsively cancel the input motion vibration. These rather crude, localized devices are not entirely adequate in producing counteractive vibratory frequencies. Additionally, like damper assemblies which utilize hydraulic fluid or which are actively controlled in some manner they are not particularly useful to control stiffness or other structural characteristics.
Damping control of structures often includes the application of a viscoelastic material to the surface of the vibrating structure. In structures where high stiffness and tensile strength with low weight is a primary factor, such as in aircraft, light weight damping materials are needed to control the excitation and transmission of bending waves in the structure. Efficient, light weight damping can often be achieved by placing a thin constraining layer over the viscoelastic material. The effect of such a layer is to enhance the amount of shear deformation experienced by the damping layer such that more energy is dissipated per cycle of vibration and damping of the vibration occurs more rapidly than would be the case without the constraining layer. This constrained layer damping treatment material can be manufactured in tape form, and is easily applied to aircraft fuselages, architectural structures and the like as a noise or vibration control measure.
A problem with constrained layer damping treatments is that they are optimized for only a single given temperature and operating frequency. The requirements for optimum damping with constrained layers are quite different than for those of a single layer where, in general, more material damping is better. In the case of a constrained layer the overall composite structure damping is a complicated function that involves both the stiffness and damping of the viscoelastic layer. Too much damping in the constrained viscoelastic layer can actually result in decreased system damping in certain frequency ranges. Variable control of the stiffness and/or damping of the constrained layer would be desirable to achieve optimum system performance over a broad frequency range.
One approach to the optimal control of constrained layer damping is that described in U.S. Pat. No. 4,565,940 where the constraining layer is a piezoelectric film. The damping effect of the constrained layer can be controlled by varying the voltage applied to the film to provide controllable damping that is somewhat effective. However, active control of vibrations using piezoelectric films, even in combination with viscoelastic materials, include various shortcomings. For example, high output piezoelectric ceramic materials are inapplicable because of their weight, their brittleness and the fact that large, thin sections are difficult to fabricate. Piezoelectric polymers, on the other hand, while light weight, flexible and available in large, thin sheets are incapable of producing forces which are sufficiently large to have a significant effect in most practical systems.
While the control of vibration and other properties of mechanical systems has been approached in a variety of ways, arrangements heretofore developed have failed to provide overall, adaptable control of structure behavior in a cost effective manner for diverse commercial application. Discrete, lumped parameter dampers which are actively controlled by electrorheological, magnetoviscous or other means for coupling between portions of mechanical systems are not well suited to global tailoring of dynamic characteristics of extended structures. On the other hand, distributed treatments such as traditional viscoelastic material layers with fixed non-controllable stiffness and damping characteristics are not well suited for global application because they are incapable of responding in an optimal fashion to changing system requirements or dealing effectively with resonant conditions. Piezoelectric augmented constrained layer treatments, while capable of improved control in certain instances where overall damping requirements are minimal, cannot provide effective control in most practical systems. The need is apparent for instantaneously controllable materials in mechanical systems which may be dynamically tailored in a distributed fashion to achieve desired performance behavior. Preferably, the structural properties to be variably controlled throughout the extent of the structure include the effective loss factor and the complex flexural rigidity of the material. Also, the materials employed should be resilient when stress and strain limits are exceeded.
It is accordingly an object of the present invention to provide flexible mechanical structures having instantaneously variable and reversible structural characteristics which eliminate or substantially minimize the above mentioned and other problems typically associated with stiffness and damping control of structures of conventional construction and operation.